1. Field of the Invention
The present invention relates generally to batteries for spacecraft applications and, more particularly, to the relocation of the battery radiators to the generally unused east and west surfaces of a body stabilized satellite. Although the term geosynchronous satellite is used throughout the disclosure, it will be understood that the invention is applicable to any orbiting satellite which maintains opposing faces normal to the sun line throughout the orbit.
2. Description of the Prior Art
The payload capacity of a commercial geosynchronous satellite may be limited by the ability of the satellite to reject waste heat. Heat rejection in space is accomplished by thermal radiation which in turn is most efficiently carried out when the radiation surface is not directly exposed to the sun. For body stabilized satellites with continuously operating payloads only the north and south facing surfaces generally meet this requirement. The other surfaces of the satellite body (east west, earth, anti earth) are all exposed, on an intermittent basis, to direct sunlight. These surfaces then heat up due to the solar flux impingement, and thus cannot be used as efficiently to dissipate heat from equipment which operates continuously due to absorbed solar radiation. The net result is a restriction in spacecraft/payload capability thermal dissipation.
This restriction is partially addressed, at present, by locating some equipment which can operate at very high temperatures on the east and west faces of the satellite. An example of such equipment is the output multiplexer (OMUX) for a communication satellite. As the OMUX is a non-electronic component, it is capable of operating at temperatures above 100.degree. C. and may thus still usefully dissipate heat even when its mounting surface is subject to direct solar illumination. The OMUX, however, does not normally require the total area of the east and west satellite faces. It is the purpose of this invention to enable the relocation of other equipment from the north/south to the east/west faces of the satellite. This relocation, in turn, will free up added north south radiator area for electronic equipment which must operate continuously and thus increase satellite capability.
The spacecraft battery represents a candidate piece satellite equipment to relocate to the east and west faces of the satellite. An obvious feature of the battery is that it produces its maximum heat when discharged; and discharge in turn usually occurs when the satellite is shadowed by the earth and there is no incident solar radiation on the east and wet surfaces (highest thermal satellite radiation capability). The battery is recharged during periods of solar illumination when alternately the east and west surfaces of the satellite are exposed to the sun. Heat rejection from the battery is required during recharge to cool the battery which has heated during the high rate discharge and reject heat generated during the inefficient periods of recharge which occur as the battery reaches a full state of charge. However, recharge need not be continuous and, in fact, for a satellite with batteries located on both the east and west faces recharge may be sequenced such that it occurs first on the shaded west face after eclipse discharge and then on the shaded east face.
The above is well recognized by those skilled in the art, and, for example the Russian Yamal satellite employs east and west located batteries combined with sequence charging in which only the shaded battery is recharged. There are, however, problems with the Yamal type implementation of east west batteries which include:
(1) The quiescent battery temperature may be adversely increased by solar radiation which will increase its temperature of operation during eclipse thereby decreasing the degree to which it can be cooled during recharge. The net result of this is that the capacity of the battery will be reduced as it may not reached the optimum recharge temperatures taught in U.S. Pat. No. 5,395,706.
(2) As the batteries are sequence charged with less than 12 hours cooling per battery the radiator area per unit of battery capacity must be larger to cool the individual batteries in the shorter time period vs. north south batteries.
(3) As the radiators are larger the power required to maintain the battery above minimum temperature is larger which adversely impacts the allocation of solar array power (typically valued at $1000 per watt for space applications) to battery heaters.
(4) Sequentially recharging batteries leads, on a two bus satellite, to a unbalanced condition between the stored energy in the two busses when the batteries recharge. This imbalance, in turn, may require shutting down the payload in an emergency loss of lock condition.
(5) If the batteries are also used as the power source for electric thrusters (used, for example, for north south stationkeeping) logistical problems may be encountered if the battery used for energy during thrust is exposed to the sun at a specified firing time.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.